MAURIZIO DE ANGELIS / SCIENCE PHOTO LIBRARY MAURIZIO DE ANGELIS / SCIENCE PHOTO LIBRARY
Fertilised egg cell dividing. Computer illustration showing the process by which a fertilised human ovum (egg cell) divides repeatedly to form a morula within the zona pellucida (outer layer). Clockwise from upper left: After fertilisation the nuclei of the sperm cell and the ovum fuse to form a zygote. The zygote then undergoes repeated mitosis (cell division). Cell division occurs approximately every twenty hours, to form a cluster of cells known as a morula, which continues to divide to form a blastocyst, and subsequently the embryo.
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